Natroalunite on Ruapehu volcano, New Zealand.

A 5 cm diameter nodule recently recovered from the northern slopes of Mount Ruapehu, Taupo Volcanic Zone, New Zealand consists of bladed natroalunite, quartz and rutile. The natroalunite crystals are zoned, having K-rich cores and Na-rich rims, and are enriched in Ba and Sr over Rb. The strontium isotopic composition (0.70535) is close to the average for Ruapehu lavas and the δ34S (‰) CDT value of +16.1 is similar to dissolved sulphate in Ruapehu crater lake. The nodule probably originated as a precipitate from hydrothermal fluids at low pH, and was subsequently baked by hot lava.

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